Mobile or cellular telephony is when an operator--such as a public department or a commercial company--provides a plurality of geographically distributed radio base stations. Each radio base station is arranged to cover a certain area or cell, and the different cells are chosen in a way that they together will cover a continuous geographic area. The radio base stations are connected via base station controllers to a switching center for mobile tele-phony, which in turn is connected to the public switched telephone network, wherein incoming and outgoing calls may be switched to and from the mobile stations present in the area, for instance radio telephones or cellular telephones. A known standard for digital cellular mobile telephony is GSM (Global System for Mobile Service), which today covers a large part of the world. In the following GSM will be used as a basis for examples and discussion, but the description below may in all essential parts be applied also to other standards for mobile tele-phony, such as for instance D-AMPS (Digital Advanced Mobile Phone System) or PDC (Pacific or Personal Digital Cellular).
When a mobile station is switched on by a user, the mobile station will search for available radio base stations and then select the particular station, which appears to have the strongest signal strength. Then the mobile station will report its presence to the selected radio base station, which will forward the information to a switching center for mobile telephony. The mobile station will then be ready to serve incoming as well as outgoing telephone calls. If the mobile station is moved a considerable distance during an ongoing call (for instance when travelling in a car), the communication will automatically be switched to another radio base station (so called hand-over). Additionally, a feature known as roaming is available (the system will detect and keep track of subscribers, which are temporarily located abroad, so that the subscriber may continue to use his mobile station also under such circumstances). Hence, a modern mobile telephone system such as GSM is able to offer the users a high-quality and highly automatized service.
The procedure above is only possible on condition that the mobile station is in operative connection with the rest of the mobile telephone system, i.e. the mobile station is in contact with a radio base station and is thus available for telephone service. Opposite situations are for instance when: a) the user has turned off the mobile station, b) the supply of power to the mobile station is insufficient (the battery has "run out"), or c) the mobile station is without reach from the radio base station--for instance in road tunnels, in subterranean locations or otherwise out of reach. For every call which does not reach the intended subscriber, the load on the mobile telephone system is increased, since the resources thereof (such as the available frequencies or channels) are unnecessarily occupied.
In order to increase the service also in such situations, where a subscriber may not be reached through his mobile station for the moment, the telephone operators offer a variety of services. One example is call-forwarding, where the subscriber may enter an alternative telephone number, to which incoming calls are to be directed, should the subscriber not be reachable on the ordinary telephone number. A further example is voice-mail services, where the caller may leave a spoken message in a voicemail box at the telephone operator and where the subscriber at a later stage may call the telephone operator in order to listen to any messages available.
A drawback of this latter service is the fact that the subscriber must himself remember to check his voicemail box at a regular basis. As an aid many operators now provide an automatic message service through so called SMS-messages (Short Messages Services), i.e. short text messages, which may be shown on the mobile station display. Once the subscriber's mobile station regains contact with the rest of the mobile telephone system, an SMS-message is sent to the mobile station with an indication that at least one spoken message is available for playback in the subscriber's voicemail box, on condition that someone actually has tried to reach the subscriber and indeed left a spoken message. The SMS-service is defined as part of the GSM standard in ETSI Technical Standards (ETS) 03.40-41. A corresponding service for American cellular telephony is defined in the EIA/TIA standard IS 136.
Despite the improvements above certain shortcomings in the procedure above may still be identified. The caller (the so called A-subscriber) must take the time to leave a spoken message to be deposited in the voicemail box of the receiving party (the B-subscriber). When the B-subscriber is reached by an SMS-message indicating that a spoken message is available, the B-subscriber must i) call his/her voicemail box, ii) take the time to listen through the message(s), and iii) in most cases locate the telephone number of the A-subscriber in question before calling back to the A-subscriber.
In EP-B-0 699 009 a mobile telephone system is illustrated, wherein the telephone number (A-ID) of the caller (A-subscriber) is automatically registered in situations, where the intended receiver of the call (B-subscriber) may not be reached for the moment. The registered telephone number (A-ID) is then generated as a text message (for instance SMS) and stored in a message central. Once the B-subscriber becomes operatively available again, the stored text message is transmitted to the mobile station of the B-subscriber, wherein the telephone number of the A-subscriber is shown on the display in the B-subscriber's mobile station. Subsequently the B-subscriber may dial the telephone number shown on the display, if desired.
The system described above has eliminated some of the shortcomings above. However, the proposed solution is not perfect, neither when it comes to user-friendliness nor with regard to the available bandwidth. In many cases a telephone number alone, i.e. a sequence of some ten digits, is insufficient for enabling the user (B-subscriber) to identify the person, which has tried to reach him/her (i.e., the A-subscriber). As a consequence the B-subscriber has to a) either find out the identity behind the A-number by for instance calling a number information service provided by the operator, or b) dial the registered A-number without a priori knowledge of the identity thereof, i.e. without being given an opportunity to prepare himself/herself and without really knowing the importance of the previous call.